Measuring the load on a fastener during installation of the fastener is a long standing problem. As a general problem, the fastener load value should be optimized depending on the fastener, assembly materials and application. During a typical application, when a fastener is put into use, it is inserted into clearance holes in a multi-part assembly. A washer and nut are lightly threaded onto the end by hand. Tightening the fastener requires two people: one person fixing the head with a standard wrench and another person on the opposite side of the structure tightening the nut with a torque wrench. The nut is tightened to a specified level of torque. In this example, the torque wrench measures torque as a proxy for bolt load. This technique suffers from inaccuracy due to inconsistent or uncalibrated friction between the fastener and its mating hole. The torque applied must overcome the contact friction as well as loading the fastener. In alternative fastener installations, one end of the fastener or bolt may be integrated into the assembly directly, however, a torque wrench measurement still suffers the same shortcomings.
For assembly in critical areas, instrumented fasteners may be used. Instrumented fasteners have a transducer bonded to the head of the fastener. In typical applications, this transducer allows an ultrasonic measurement of the fastener extension, using a commercially available instrument for ultrasonic fastener load measurement. In available commercial ultrasonic instruments, ultrasonic vibration is induced mechanically at the transducer end of the fastener. In a typical use, the instrument then measures the arrival time of the induced wave that travels the length of the fastener, reflects from the opposite face and returns to the starting point. As the fastener is tightened to apply tensional stress, the fastener is put under positive strain and increases in length. At the same time, the sound velocity decreases through the acousto-elastic effect. An ultrasonic pulse propagating through a loaded fastener thus propagates a greater distance at a slower velocity than in an unloaded fastener, producing a time delay that can be used to determine the internal stress in the shank of the fastener.
In a maintenance or service environment it is desirable to measure the current load. However, there is no measurement reference point at zero load, so the transducer approach cannot be used. The use of applied torque for load measurement suffers from the same limitations described earlier.
Current commercial devices which perform transducer-based measurement of fastener extension have several problems including difficult, complex and cumbersome use due in part to the transducer-contact interface, cost, a limitation of the accuracy of the device and that the measurements were available in real time.